Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse-electrical (TE) and transverse-magnetic (TM) modes are generally different. Polarization-dependent dispersion and polarization-dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon-on-insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.

Polarization management for silicon photonic integrated circuits

Silicon photonics has experienced phenomenal transformations over the last decade. In this paper, we present some of the notable advances in silicon-based passive and active optical interconnect components, and highlight some of our key contributions. Light is also cast on few other parallel technologies that are working in tandem with silicon-based structures, and providing unique functions not achievable with any single system acting alone. With an increasing utilization of CMOS foundries for silicon photonics fabrication, a viable path for realizing extremely low-cost integrated optoelectronics has been paved. These advances are expected to benefit several application domains in the years to come, including communication networks, sensing, and nonlinear systems.

Recent advances in silicon-based passive and active optical interconnects

We demonstrate a compact, high speed Ge photodetector efficiently butt-coupled with a large cross-section silicon-on-insulate (SOI) waveguide in which the Ge p-i-n junction is placed in the horizontal direction to enable very high speed operation. The demonstrated photodetector has an active area of only 0.8×10 μm2, greater than 32 GHz optical bandwidth, and a responsivity of 1.1 A/W at a wavelength of 1550 nm. Very importantly the device can readily be integrated with high performance wavelength-division-multiplexing filters based on large cross-section SOI waveguide to form monolithic integrated silicon photonics receivers for multichannel terabit data transmission applications.

High-speed Ge photodetector monolithically integrated with large cross-section silicon-on-insulator waveguide

We demonstrate a compact waveguide-based high-speed Ge electro-absorption (EA) modulator integrated with a single mode 3 µm silicon-on-isolator (SOI) waveguide. The Ge EA modulator is based on a horizontally-oriented p-i-n structure butt-coupled with a deep-etched silicon waveguide, which transitions adiabatically to a shallow-etched single mode large core SOI waveguide. The demonstrated device has a compact active region of 1.0 × 45 µm(2), a total insertion loss of 2.5-5 dB and an extinction ratio of 4-7.5 dB over a wavelength range of 1610-1640 nm with -4V(pp) bias. The estimated Δα/α value is in the range of 2-3.3. The 3 dB bandwidth measurements show that the device is capable of operating at more than 30 GHz. Clear eye-diagram openings at 12.5 Gbps demonstrates large signal modulation at high transmission rate.

30GHz Ge electro-absorption modulator integrated with 3 μm silicon-on-insulator waveguide.

Wavelength-division-multiplexing (WDM) networks with wide channel grids and bandwidths are promising for low-cost, low-power optical interconnects. Wide-bandwidth, single-band (i.e., no free-spectral range) add-drop filters have been developed on silicon using anti-reflection contra-directional couplers with out-of-phase Bragg gratings. Using such filter components, we demonstrate a 4-channel, coarse-WDM demultiplexer with flat passbands of up to 13 nm and an ultra-compact size of 1.2 × 10−3 mm2.

Ultra-compact, flat-top demultiplexer using anti-reflection contra-directional couplers for CWDM networks on silicon.

We demonstrate a compact, low crosstalk, low loss, and flat passband demultiplexer based on an echelle grating fabricated in the silicon-on-insulator (SOI) platform. The demonstrated 12 channel demultiplexer has an 8-nm channel spacing, 5.5-nm flat passband, 1.7-dB on-chip loss, and better than 25-dB optical crosstalk. By arranging the output waveguides very close to the zero degree angle of the echelle grating, the performance of the demonstrated device is made insensitive to the vertical angle of the grating facet. The fabricated devices have a very small footprint and have the potential to provide a low-cost demultiplexer solution for multichannel data transmission applications.

Fabrication Insensitive Echelle Grating in Silicon-on-Insulator Platform

A novel design of a silicon-on-insulator (SOI) filter for passive optical network applications is described. The SOI filter comprises a monolithically integrated planar reflective grating and a multistage Mach-Zehnder interferometer. The fabricated device showed low insertion losses and high optical isolation. To the best of our knowledge, this letter describes the first demonstration of an SOI circuit comprising monolithically integrated planar reflective grating and cascaded Mach-Zehnder interferometer

Silicon-on-Insulator-Based Planar Circuit for Passive Optical Network Applications

We report 10Gb/s optical-proximity-communication with reflecting mirrors micro-machined into Si and co-integrated to low loss SOI waveguides for chip to chip communication using a self-aligned-packaging mechanism with measured 4.0 dB coupling loss.

Optical Proximity Communication in packaged SiPhotonics

We have designed and fabricated a low-loss polarization-insensitive silicon-on-insulator-based photonic integrated circuit wavelength-division-multiplexing transceiver filter for triplexer applications. The filter is realized by two-stage Fourier-transform-based Mach-Zehnder interferometers to handle the uploading of 1.3-mum data stream and the demultiplexing of 1.49-mum/1.55-mum channels for downstream traffics, respectively. The fabricated filter has less than 0.2-dB ripple over 20-nm flat passband for downstream channels and more than 100-nm bandwidth for the upstream channel. The excess loss of the filter is less than 0.35 dB with less than 0.1-dB polarization-dependent loss.

Low-Loss Polarization-Insensitive Silicon-on-Insulator-Based WDM Filter for Triplexer Applications

Flat-top demultiplexers using etched diffraction gratings in the silicon-on-insulator (SOI) platform are reported and experimentally demonstrated. The novel design enables the loss and crosstalk of the devices to be insensitive to the vertical angle of the grating facets.

Cheng-Chih Kung

Papers

Fabrication Insensitive Echelle Grating in Silicon-on-Insulator Platform

Silicon-on-Insulator-Based Planar Circuit for Passive Optical Network Applications

Optical Proximity Communication in packaged SiPhotonics

Low-Loss Polarization-Insensitive Silicon-on-Insulator-Based WDM Filter for Triplexer Applications

Novel fabrication tolerant flat-top demultiplexers based on etched diffraction gratings in SOI